The fluid catalytic cracking (FCC) process is well known for conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons. In many catalytic cracking reactors, often referred to as riser reactors, risers or pipe reactors, a long chain hydrocarbon feed reacts with a catalyst to produce shorter chain products. This can be referred to as cracking the feed. The feed and fluidized catalyst are introduced at a lower entrance to the vertical riser, and travel vertically upwards within the riser reacting at very high temperatures until they reach an upper exit. The riser is often internally lined to minimize heat loss and resist erosion/corrosion.
Reaction efficiency in the riser depends, among other factors, on good and uniform mixing between the feedstock and fluidized catalyst. It is desirable that the feed be uniformly dispersed in a stream of fluidized catalyst that is moving up the riser. In many risers, however, even if near uniform dispersion is achieved at the riser entrance, non-uniform mixing can occur as the materials travel upwards due (at least in part) to non-uniform cross sectional gas velocities that result from temperature differentials and other factors. In some risers, for example, the upward velocity of feedstock is lower near the riser wall and higher near the center. This non-uniform velocity profile may be referred to as reactor slip. Under such conditions, more dense fluidized catalyst tends to concentrate near the wall in the slower velocity feedstock. This leads to lowered reaction efficiency and yield.
Some attempts have been made to improve mixing along the vertical flow path of the riser. For example, obstacles such as baffles or other contact devices have been proposed to create turbulence and cause more uniform mixing in the riser. However, proposals to date have suffered various problems and disadvantages. Many relate to erosion and/or corrosion. The riser creates a highly corrosive and erosive environment that combines high temperatures and a high flow rate of chemically active materials. Other problems relate to temperature differentials of contact devices. Metal baffles and the like are subject to temperature gradients along their length. Such gradients can lead to mechanical stresses and failures and can even lead to condensation of reactants on the baffle in extreme cases.